Electricity meters are widely used for measuring electrical energy consumption. FIG. 1 shows a diagram of an exemplary conventional electricity meter 10 configured to measure power flowing the power lines 9 to a facility. The electricity meter 10 includes an input circuit 11 configured to sense current and voltage on the power lines 9. The electricity meter 10 also includes a processing circuit 12 that is configured to receive the sensed current and voltage from the input circuit 11 and provide information representative of energy consumption to various outputs. The outputs may include a meter display 14 and an automatic meter reading (AMR) communications module 15. Another output that may be provided in conventional electricity meters is the optical communications port 16, which allows for optical communications to and from the processing circuit 12. Yet another output that is provided on many conventional electricity meters is the pulse wave output 17, which is configured to be hard-wired to an external device 18 that is housed completely separate from the meter 10.
The pulse wave output 17 is configured to receive output pulses from the processing circuit 12 that represent an energy metric measured by the meter 10. For example, it is known for a meter to generate so-called “KYZ pulses” that are representative of energy consumption. FIG. 2a shows an exemplary “alternating state” pulse train 20 that is representative of a KY or KZ pulse waveform. The waveform consists of a series of pulses 21, 22, 23, 24, 25, 26 and 27 wherein each pulse merely changes the signal logic state and remains in that logic state until the next pulse. In a KY or KZ pulse waveform, each change in state (e.g., on to off/high to low) is representative of a quantity of electrical energy consumed. FIG. 2b shows a differential alternating state pulse train 25 which includes two identical pulse trains 26 and 27 having opposite polarity. Such a differential pulse train 25 is used in many meters and is often referred to as a “KYZ pulse signal”.
With reference again to FIG. 1, the pulse wave output is provided by a KYZ output/input card 17 or other interface/device configured to deliver a KYZ pulse signal (such as that shown in FIG. 2a or 2b). It is known to use the output pulses delivered from the electricity meter's pulse wave output 17 in a device external to the meter for various purposes. For example, the external device 18 may use the output pulses of a KYZ pulse (or other pulse) for load control, a rate indicator, an end-of-interval, or for other kinds of control inputs as will be recognized by those of skill in the art. Accordingly, exemplary external devices 18 include load control devices, rate indicators, pulse recorders or the like.
As mentioned above, in current implementation, the pulse wave output 17 of the meter is wired to a receiver of an external device 18, such as a building control system device or other output device that is external to the meter. One drawback of the wired connection between the pulse wave output 17 of the meter 10 and the external device 18 is the difficulty of protecting the interface for the pulse wave output 17 in the meter 10 and the receiver at the external device 18 from high voltage spikes when the electrical system is excited by high voltage from equipment operations or lightening strikes. In particular, some metering systems are connected to ungrounded electrical service, and can experience voltage surges in excess of a few thousand volts. It is not uncommon for these voltage spikes to exceed the 4 KV isolation provided in the equipment. When the meter 10 is completely isolated from ground, with no connection to a grounded external device 18, such spikes are not necessarily problematic. However, when the meter 10 is wired to an external device 18 having a local ground, problems can exist. In particular, because of the wiring between the pulse wave output 17 of the meter 10 and the receiver of the external device 18, a path to ground is provided via the external devices 18. Thus, large voltage spikes in an ungrounded meter 10 may result in damage to the equipment, including damage to the processor 12, the interface of the pulse wave output 17, the receiver of the external device 18, and other equipment.
In view of the above, it would be advantageous to provide an arrangement for measuring electrical energy consumption where an ungrounded electricity meter is protected from voltage spikes.